1. Field of the Invention
The present invention relates to a vibratory separation system and a membrane module and other components which may be used in a vibratory separation system.
2. Discussion of the Prior Art
Separation devices are typically utilized to separate one or more components of a fluid from other components in the fluid. As used herein, the term "fluid" includes liquids, gases, and mixtures and combinations of liquids, gases and/or solids. A wide variety of common processes are carried out in separation devices, including, for example, classic or particle filtration, microfiltration, ultrafiltration, nanofiltration, reverse osmosis (hyperfiltration), dialysis, electrodialysis, prevaporation, water splitting, sieving, affinity separation, affinity purification, affinity sorption, chromatography, gel filtration, bacteriological filtration, and coalescence. Typical separation devices may include dead end filters, open end filters, cross-flow filters, dynamic filters, vibratory separation filters, disposable filters, regenerable filters including backwashable, blowback and solvent cleanable, and hybrid filters which comprise different aspects of the various above described devices.
Accordingly, as used herein, the term "separation" shall be understood to include all processes, including filtration, wherein one or more components of a fluid is or are separated from the other components of the fluid. The term "filter" shall be understood to include any medium made of any material that allows one or more components of a fluid to pass therethrough in order to separate those components from the other components of the fluid. The terminology utilized to define the various components of the fluid undergoing separation and the products of these processes may vary widely depending upon the application, e.g., liquid or gas filtration, and the type of separation system utilized, e.g., dead end or open end systems; however, for clarity, the following terms shall be utilized. The fluid which is input to the separation system shall be referred to as process fluid and construed to include any fluid undergoing separation. The portion of the fluid which passes through the separation medium shall be referred to as permeate and construed to include filtrate as well as other terms. The portion of the fluid which does not pass through the separation medium shall be referred to as retentate and construed to include concentrate, bleed fluid, as well as other terms.
A common problem in virtually all separation systems is blinding or fouling of the filter, for example, a permeable membrane. Permeate passing through the filter from the upstream side to the downstream side of the filter leaves a retentate layer adjacent to the upstream side of the filter having a different composition than that of the process fluid. This retentate layer may include components which bind to the filter and clog its pores, thereby fouling the filter, or may remain as a stagnant boundary layer, either of which hinders transport of the components trying to pass through the filter to the downstream side of the filter. In essence, mass transport through the filter per unit time, i.e., flux, may be reduced and the inherent sieving or trapping capability of the filter may be adversely affected.
In certain filter systems, it is well known that if the filter and the layer of fluid adjacent to the surface of the filter are moved rapidly with respect to each other, fouling of the filter is greatly reduced. Accordingly, filter life is prolonged and permeate flow rate is improved. Essentially, the two categories of separation technology which are currently utilized for developing relative motion between the fluid and the filter are cross flow filter systems and dynamic filter systems.
In cross flow systems, high volumes of fluid are typically driven through narrow passages bounded by the filter surface and possibly the inner surface of the filter housing, thereby creating the preferred movement of fluid across the filter. For example, process fluid may be pumped across the upstream surface of the filter at a velocity high enough to disrupt and back mix the boundary layer. An inherent weakness common to cross flow filter systems is that a significant pressure drop occurs between the inlet and outlet of the filter system. Specifically, the process fluid entering the filter system is under a great deal of pressure in order to develop high flow velocities; however, as the process fluid is dispersed tangentially across the upstream surface of the filter, the pressure sharply decreases. This decrease in pressure tangentially across the upstream surface of the filter causes non-uniformity in transmembrane pressure, i.e., the pressure difference through the filter between the upstream and downstream sides of the filter. This non-uniformity in transmembrane pressure tends to increase fouling of the filter. Accordingly, filter longevity and efficiency is reduced because certain areas of the filter may become fouled more rapidly than other areas. Additionally, this makes the scaling up of cross flow systems difficult. Generally, filter systems are scaled up by adding additional filter elements, but adding filter elements increases the pressure differential and induces greater non-uniformity.
Further, many components in process fluids cannot withstand the high flow rates used in cross flow filter systems. For example, the maximum allowable velocity for many biological fluids is far too low to allow adequate back-mixing and thereby reduce or eliminate the stagnant boundary layer. Furthermore, the required high feed rates as compared to the filtration rates in cross flow systems require numerous feed recycles through the system, which are also undesirable.
Dynamic filter systems overcome many of the problems associated with cross flow filter systems by driving a movable structural element, such as a rotatable element, adjacent to the fluid rather than using a large pressure differential to drive the fluid across the surface of a filter. Dynamic filter systems may be constructed in various configurations. Two widely used configurations are cylinder devices and disc devices. Within each of these two configurations, numerous variations in design exist.
In cylinder devices, a cylindrical filter element is positioned concentrically next to a cylindrical shell or filter housing. The process fluid is introduced into the gap between the filter element and the shell, and either the filter element or the shell is rotated about a common axis. While the filter element or the shell is rotating one or more components of the process fluid in the gap pass through the filter element and are recovered as permeate. Cylindrical devices are highly efficient because rotating the filter element or the shell with respect to the process fluid in the gap greatly reduces fouling of the filter element. However, due to manufacturing and operational limitations, cylindrical devices cannot be made large, e.g., it is difficult to increase filter surface area because of constraints on the diameter of the filter element.
In disc devices a set of disc-shaped filter elements are stacked in parallel along a common axis and positioned within the filter housing. In these devices the fluid motion is created by rotating the filter discs, or by rotating a set of impermeable discs which are interleaved between the filter discs. Disc devices overcome some of the disadvantages of cross flow and cylinder devices but suffer from complexly of design. Further, while the ratio of the filter surface area to the housing volume in a disc device may be superior to that of a cylinder device, the ratio is still relatively low.
A common concern in many conventional dynamic filter systems is the high energy requirement for effective filtration. Typically, in rotating devices, the energy requirement may be quite high. Specifically, significant energy may be utilized to overcome the high moment of inertia of the rotating portion of the system, as well as maintaining the high rotation rates. Another concern associated with dynamic filter systems is non-uniformity in transmembrane pressure. In rotating systems, certain conditions may result in fluid dynamics that produce non-uniform transmembrane pressure which may cause preferential fouling of the filter. These conditions generally occur in the filtration of highly viscous fluids and fluids containing high concentrations of solids.
Another disadvantage associated with some conventional dynamic filter systems is that they are very difficult to clean in place, i.e., to clean without completely disassembling the system. A conventional dynamic system typically has a multi-component housing, filter unit, and rotational unit, each of which may be rife with cracks and crevices. Further, the filter unit and the rotational unit are frequently constructed and positioned within the housing in a manner which results in stagnant regions or regions of low flow velocity within the housing. These cracks, crevices, stagnant regions, and low flow velocity regions all collect and harbor contaminants which may be difficult or impossible to remove by cleaning in place. In addition, O-rings and similar seals present barriers to the flow of fluid and are thus collection areas for contaminants.
Vibratory dynamic filter systems in which the filter discs are oscillated at predetermined frequencies are also well known as is seen from an examination of the pertinent patent art. U.S. Pat. No. 4,526,688, for example, proposes a shock-type system where the membrane support structure and a filtration apparatus are periodically banged to induce the filter cake to drop from the filter. U.S. Pat. No. 4,545,969 employs a shearing plate which is oscillated parallel to a fixed filter. U.S. Pat. No. 3,970,564 discloses a system where a filter is mechanically vibrated in a direction normal to the filter. Vibrations have also been created using ultrasonic transducers such as those found in U.S. Pat. No. 4,253,962.
Typically, in vibratory dynamic filtration systems a tradeoff between filter surface area and system weight must be made. Increased surface area for filtration is always desired; however, increasing surface area usually involves increasing the overall weight of the filtration system. Weight is generally a problem for all filtration systems, due for example, to size and transportability constraints, but is of particular importance in vibratory filtration systems. As the weight of an object increases, so does its moment of inertia. Accordingly, increased weight in vibratory filtration systems means that the vibratory drives of these systems must be larger and require additional energy to overcome the increased moments of inertia, and are thereby less efficient. The current state of the art vibratory filtration system has not adequately resolved the surface area--weight tradeoff. For example, typical vibratory filtration systems comprise large, high volume housings, which are not of inconsequential weight. These systems also have low ratios of filter surface area to housing volume.